Many “Alternative” Energy Sources (AES), such as photovoltaic (PV) modules, produce maximum power at a DC output voltage that varies widely depending upon the solar insolation levels, ambient temperature and other variables. Wind energy, which is often extracted to a DC output using a wind generator (WG) with a rectifier, generally also requires a variable output voltage to extract the maximum power at any given time or wind speed. It is important to operate a PV or WG system at the DC voltage at which maximum power is obtained from these sources, so as to obtain the maximum benefit from the equipment capital expenditure. Since the DC voltage must vary, some form of power conversion is required to transfer energy from the source to a battery whose voltage is independently determined. Typically, a charge controller is used to transfer power from the PV or WG to a battery in a parallel configuration. The power from the battery is then converted to AC using an inverter to energise AC loads.
Referring to FIG. 1, a common topology for a power conversion system 10 is the series connection of a DC energy source S, for example a PV or WG, to a battery charger 12, battery 14 and an inverter 16. The inverter 16 provides an AC output for an AC load 18.
Power conversion systems are often manifested UPS systems. Recently, the application of PV assisted UPS systems for poor quality utility power grids has been reported, where a bi-directional inverter is used in an “in-line” configuration as shown in FIG. 2. In this configuration, as with the series configuration in FIG. 1, the battery charger 12 has to carry the full power of source S, regardless of whether the energy is intended to flow entirely into the battery 14, or out to the AC grid 20. Energy not intended for the battery 14 must then be converted again, resulting in a system where the cost and efficiency has not been optimised.
This system consists essentially of three energy sources (where a source could be a load, or negative source). The first is the DC source S itself, which supplies energy when available. The second is the battery 14, which acts as energy storage, accepting energy from the source S or the AC grid 20 at certain periods of time, and supplying energy to the AC grid 20 when energy is not available from the DC source S. The third energy source is the AC grid 20 itself, which could accept energy from the DC source S or the battery 14, or provide energy either to charge the battery 14 or supply loads 18.
In this system, a topological arrangement of power conversion equipment is required to provide all possible power flow requirements as efficiently as possible with the lowest aggregate converter power rating.
A single conversion between each of the three sources would have the greatest efficiency, since only one converter would be required for each conversion. However, this would require three converters, each with full power rating.
Throughout this specification and claims the terms “converter”, “rectifier”, “inverter” and “battery” are intended to have the following meaning, unless from the context of their use it is clearly apparent that an alternate meaning is intended:    Converter: any device which can convert power from AC to DC, or DC to AC uni-directionally or bi-directionally. Thus the term “converter” includes within its scope an inverter and a rectifier.    Rectifier: any device which converts AC power to DC power.    Inverter: any device which converts DC power to AC power.    Bi-directional Inverter: any device which converts AC power to DC power and DC power to AC power.    Battery any energy storage device comprising either a battery by itself, or any other type of energy storage device, or any energy storage device in combination with a second alternate energy source which has energy storage properties such as a fuel cell